Rock Structures and Formations

There are all kinds of shapes in rock bodies, but the shapes are all variations on a few basic forms. Volcanic and sedimentary rocks all start out as flat beds or layers, which can later be deformed by the continuing motion of the surface of the Earth. If the layers are tilted, a person who studies geology can describe it as the orientation of the layer in terms of its dip and strike.

The dip and stirke of an orebody can be found similar to a sheet of plywood sitting on an angle in a tank of water. The line where the surface of the water and the sheet intersect is the line of strike – it is a horizontal line with a bearing that can be measured with the help of a compass. The angle that the wood makes with the surface of the water is its dip. Both dip and strike are most of the time measured in degrees.

Folds are caused by sideways pressure on flat-lying bedded rocks which buckle and form a pattern which is wavy. If you bend a thin book slightly, and if you view the top edges of the pages, you will see that it gives you an idea of the structure of a fold. Folds with “trough” shapes are called synclines, and the folds with “crest” shapes are called anticlines.

Intrusive rocks most of the time form large bodies called batholiths or smaller, pipe-like bodies called stocks. A dyke is an intrusive body with the shape of a sheet that cuts through the surrounding country rock. A sill also has the shape of a sheet, but it forms along a space between bedded rocks. Dykes and sills have a dip and a strike like the bedded rocks.

Something that is very common in rock are the fractures. If the fracture is large enough, and the rocks have been torn apart, the fracture is called a fault. If the fault is not just one fracture, but instead it is a series of of many diminute parallel fractures, it goes by the name of a shear zone.

Shear zones, fractures and faults, like dykes and beds, are planar features that can be described by their dip and strike. It is also of much use to label the upper surface of an inclined fault as the hangingwall, and the lower surface as the footwall.

For a geologist it is important to know the sense of movement along the fault – upward, downward or sideways – because this permits rock units which have been broken apart by the fault to be traced. For instance, if an orebody is broken apart by later faulting, the fact of knowing whether the hangingwall has moved up or down against the footwall makes it feasible for a geologist to predict where the rest of the orebody is likely to be.

Fracturing can also help to form orebodies. Openings and fractures in the rock permit fluids to pass through, and the dissolved material in the fluids can be left behind to form a vein.

Some structures have more of a linear shape – for example, a pipelike intrusive body, the axis of a fold, or an ore shoot in a vein. The orientation of a liner structure is much better described by the trend it has, what direction iit is heading towards, and the plunge it will take, which is the angle it makes with the surface of the Earth. It is very common to speak of a structure as “plunging gently to the southwest” or “steeply to the north”, which explains only that it descends in that general direction; for more accurate applications, we describe the plunge and trend expressed in degrees.